I'll try to be clearer since I'm causing confusion. (And Ike and Roy absolutely are based on the same Marth special moveset, which is the "soul" of a character in my book. There's no need for Marth, Ike, Roy, and Lucina to all co-exist. It's different tires on the same bicycle. Like I said, stuff about the damage gradient on the swords or hitboxes or whatever is missing the point. But we're setting that aside!)
-Is it not true that Melee remains significantly more popular than any other version of the game for tournaments? Is the Street Fighter II scene bigger than SF V?
-Regardless of competitive stuff, the base game is stale. Squeezing in an increasingly marginal extra 15 characters into the same Smash4 framework is a diminishing returns proposition. I'd rather they take another crack at reinventing the no-combo-input fighting genre, and/or rebuild a smaller core set of characters from the ground up to make things fresh instead of having 50 characters that are significantly worse than (or redundant with) 25 other characters (and hey, that could even help the competitive stuff too!). I'd personally prefer novel mechanics over maximal fan service. And if what you say is true, this should be fine because the games are all totally separate from and coeval with each other!
-Also, having 75 characters makes it less likely they'll have meaningful single player challenges and such. Smash4 was quite disappointing in this respect already. I want the targets back!
-Some random ideas: Incorporate counters into shielding or dodging or something instead of a bunch of characters having it as a special move. Remix the clones into the custom move system so there are single roster slots that you can change into variants like you do with costumes now.
But again, just spitballing. Hopefully they've got some surprises in store. Even just adding stock online modes for randos and having the option to remove dynamic elements like the yellow Mega Man monster from stages would be an improvement.
Melee's competitive scene is probably larger, yeah, but I still don't understand the relevance of bringing that up. You could make the next Smash Bros. game better balanced, give it more interesting mechanics, or do just about anything, and most Melee players would just keep playing Melee because that's the only game they want to play (and the scene would most likely remain larger as a result). If you look at Smash 4's competitive scene on its own, it was at EVO Japan 2018, will be at the main EVO tournament for the fourth straight year, and has large tournaments taking place regularly. The fact that Melee's competitive scene is larger just says that it's even more successful and, as I alluded to earlier, Nintendo is not going to convert that fanbase no matter how good or how different the next Smash Bros. game is.
When you start talking about significantly altering the gameplay or rebuilding from the ground up, you might as well make a new series at that point. If you're going to greatly change the mechanics and focus on a smaller set of characters that are largely distinct from their current Smash Bros. iterations, you've basically rebooted the series. Maybe that's what you want and what you're trying to get at, but given that most fans are happy with the core gameplay in its current form (with most complaints being levied at stuff like single-player content or online modes) and the fact that the series continues to enjoy great critical and commercial success, I don't think that's a path Nintendo really needs to follow.